Over time, fuel tanks may become contaminated as contaminants are introduced into a fuel system. Fuel tank contaminants may include any solid, liquid or gas which is undesirable in a fuel tank environment. Some examples of common contaminants in a fuel tank include water (both dissolved and free or suspended), ice, microbial or fungal contamination, oil, debris and gaseous contaminants such as oxygen, hydrogen and CO2.
Contaminants may be introduced into a fuel tank via its vent system. Alternatively contaminants may be introduced through an inlet when fuel is introduced into the tank. Contaminants may accumulate in stationary tanks, for example storage tanks or generator fuel tanks, or in vehicle fuel tanks, for example fuel tanks in an aircraft, boat, train, car, etc.
The accumulation of contaminants in a fuel tank over time may lead to a need to monitor contamination levels to verify that the quantity of a contaminant does not exceed an acceptable limit, e.g. a performance threshold, and may periodically require action to be taken to reduce the quantity of the contaminant in the fuel tank. For example, moisture may enter an aircraft fuel tank during refueling and via its vent system, resulting in a need to periodically drain water from the fuel tank.
Fuel tank contamination levels may be measured directly at predetermined intervals to establish whether or not action should be taken to reduce contamination levels, for example by establishing whether or not the quantity of a contaminant in a fuel tank has reached an action level. Alternatively, action may be taken at predetermined intervals to reduce the contamination level regardless of the quantity of the contaminant present.
In the case of an aircraft fuel tank, a water drain maintenance activity is typically performed after a predetermined number of flight hours, or a predetermined number of flights, or a predetermined number of flights in a particular sector, e.g. tropics, arctic, etc., to account for the significant variation in the ambient humidity during the descent phase of a flight which affects the ingress of water via the fuel tank vent during descent.
Since the rate of accumulation of contaminants within a fuel tank is unlikely to be constant, and may be affected by one or more operating conditions, the intervals at which action should be taken in response to the accumulation of a contaminant is also unlikely to be constant. If maintenance tasks are conducted at predetermined intervals, this may result in unnecessary maintenance activities when a contaminant has accumulated at a slower than expected rate, which will increase operating costs and lead to unnecessary down time of the fuel system. In the case of a vehicle, for example an aircraft, this may require the vehicle to be out of service for longer than is necessary. In addition, time-based maintenance is ineffective at identifying problems which may develop between scheduled inspections.
In some industries, a technique called predictive maintenance (PdM) is used to decide when maintenance activities should occur. PdM tracks the performance or condition of an asset over time and uses this information to determine when maintenance activities will need to occur, therefore allowing maintenance activities to be planned in a cost and time effective manner. PdM can reduce costs and equipment down time compared to a conventional time-based and/or operation count-based maintenance approach.